Jarosite dissolution rates and maximum lifetimes in high salinity brines: Implications for Earth and Mars

Jarosite is a ferric sulfate salt ((K, H, Na)Fe3(SO4)2(OH)6) that forms in acidic, oxidizing environments on Earth and has also been observed in outcrops on Mars. High chloride concentrations within the outcrops at Meridiani Planum suggest that jarosite likely interacted with high salinity brines. This study examines jarosite dissolution in H2O–CaCl2, and H2O–NaCl brines (activity of water, aH2O=0.35 and 0.75 respectively) to determine the effects of high salinity brines and aH2O on jarosite dissolution rates. Within brines with aH2O=0.75 and 0.35, initial K-jarosite dissolution rates at 298 K decrease from log r=−9.9 to −11.6 mol m−2 s−1, and Na-jarosite rates decrease from log r=−10.6 to −11.2 mol m−2 s−1, respectively. In addition, K-jarosite dissolution in NaCl brine at 263 K yielded an average dissolution rate of log r=−11.6 mol m−2 s−1. Applying a shrinking sphere model to determine 1 mm jarosite particle lifetimes extends the maximum duration of fluid alteration from lifetimes of <500 years calculated for dilute solutions up to 30,000+ years in cold, high salinity conditions. While reduced activity of water in high salinity systems decreases the initial rate of jarosite dissolution, increased activity of chloride ions and water in solution due to sulfate precipitation effectively increased the jarosite dissolution rate over days to weeks. This suggests that jarosite dissolution rates increase with time within eutectic brines, perhaps due to Cl− attack on residual Fe3+ left on the surface of jarosite grains. If brines on Mars became highly concentrated in chlorine ions through sulfate precipitation, the dissolution rate of jarosite, and perhaps other minerals as well, could accelerate with time, shortening particle lifetimes and the inferred duration of aqueous diagenesis significantly.